It might depend on country, but typically it's 30 MILES per hour or 50 KILOMETERS per hour, which are roughly equivalent and match default speed limits on most European roads.

45 km/h I think. 50cc and/or 4 kW depending on type of motor.

But basically with light driver this isn't too hard enforcement.

How does that work when going downhill?

30km/h at the crest can easily turn into 60+ km/h at the bottom if it's steep.